Continuous bias control for electrographic development apparatus

ABSTRACT

Apparatus for use in an electrographic development station having a long development zone, for example, having two magnetic brushes mounted in tandem. The apparatus continuously senses an electrostatic charge pattern and applies an electrical bias to the development station which corresponds to the intensity of the sensed charge. The application of the bias is delayed until the charge pattern passes the last development portion of the zone, for example, the second magnetic brush. In a preferred embodiment, the charge pattern is sensed by an induction plate coupled to an amplifier which provides a signal to a Pade&#39;&#39; pure time delay circuit. The delayed signal drives a high voltage bias amplifier which accordingly biases the development station. The delay is chosen to correspond to the time it takes the sensed pattern to pass the later portion of the development zone.

United States Patent Fearnside 1 1 Nov. 11, 1975 [75] Inventor: William T. Fearnside, Fishers. NY.

[73] Assignee: Eastman Kodak Company,

Rochester. NY.

[22] Filed: Mar. 4, 1974 [21] Appl. Nos-147,717

[52] US. Cl. 118/8; 118/637; 427/18 [51] Int. Cl. G03C 15/08 [58] Field of Search. 118/8. 9. ll. 4. 637. D16. 23: 117/175; 355/14; 427/18. 20

[56] References Cited UNlTED STATES PATENTS 3.554.161 1/1971 Blanchette 1114/14 3.611.982 10/1971 Coriale ct a1. 118/4 3.654.893 4/1972 Piper et a1. 118/8 3.674.532 7/1972 Morse 118/8 3.779.204 12/1973 Altmann 118/8 Primary E.\'umi11erMervin Stein Asxsisuml Evan1inerDoug1as Salser Attorney. Agent. 01' F1'1'/11-C. W. Williams [57] ABSTRACT Apparatus for use in an electrographic development station having a long development zone. for example, having two magnetic brushes mounted in tandem. The apparatus continuously senses an electrostatic charge pattern and applies an electrical bias to the development station which corresponds to the intensity of the sensed charge. The application of the bias is delayed until the charge pattern passes the last development portion of the zone. for example. the second magnetic brush. In a preferred embodiment. the charge pattern is sensed by an induction plate coupled to an amplifier which provides a signal to a Pade pure time delay circuit. The delayed signal drives a high voltage bias amplifier which accordingly biases the development station. The delay is chosen to correspond to the time it takes the sensed pattern to pass the later portion of the development zone.

9 Claims. 1 Drawing Figure PADE' DELAY CIRCUIT 1 MPL IFIER U.S. Patent Nov. 11, 1975 32% mmtlmq 53mm SE w 9 Fa mm r m V.

CONTINUOUS BIAS CONTROL FOR ELECTROGRAPHIC DEVELOPMENT APPARATUS Background of the Invention This invention relates to electrographic development and more particularly to an apparatus for automatically adjusting the electrical bias applied to a development electrode to compensate for variations in an electrostatic image being developed.

Description of the Prior Art In developing electrostatic charge patterns or images carried by a surface, it is common to apply to it finelydivided toner attracted to the charge pattern. For example, the surface can be contacted with a magnetic developer brush, the bristles of which comprise a mixture of toner and metallic carrier particles. Upon contacting the surface, the toner particles, which are charged to a polarity to be attracted to the electrostatic charge pattern, are selectively deposited on the surface in accordance with the charge pattern.

Recent developments have lessened the time in which the latent electrostatic image would be in contact with most development apparatus because of the increased linear speed of the pattern bearing surface. This in turn decreases the amount of fresh toner that can be brought in contact with or presented to the pattern. In order to increase the amount of toner presented to the pattern, it is common to increase the length of the development zone. One approach to increasing the length of the development zone has been to position two or more magnetic brushes close together in tandem in the same developer housing and adjacent the pattern bearing surface. The magnetic fields produced by the brushes convey developer material into the space between the brushes, giving continuous contact with the electrical charges on the pattern bearing surface over a broad area encompassing both brushes and the space between them, see for example, US. Pat. No. 3,543,720, Drexler et al.

In order to obtain solid area development, some development apparatus has included a "development electrode. A development electrode is an at least partially conductive member arranged in close proximity to the recording element during the development process. It is generally equipotential and biased, as described below. The primary function of the development electrode is to assist development of large solid areas of electrostatic image by creating electric fields from the electrode toward such areas. The strength of such fields is proportional to the charge intensity in such areas and the electrical potential of the electrode relative to the recording element. These fields cause toner particles to move toward and adhere to the recording element during the development period. In a magnetic brush, the development electrode is generally the brush itself, that is, the carrier particles. The bias voltage can be applied in many cases to the container for the carrier particles.

While such a system does give solid area development, it frequently has a tendency to develop poor contrast and unclear backgrounds from a number of operating variables that can vary the charge pattern. The poor results are due to a reluctance of the residual toner to leave the background areas when subjected to an ordinary toning operation using a development electrode, even where two or more magnetic brushes are used.

To clear up this background, the art has carefully adjusted the electrical potential of the electrode so that the background areas of the electrostatic image can be made electrostatically unattractive and even repulsive to toner particles, while the image is attractive to toner. The result is copies having low density background areas, i.e., background areas substantially free of toner. To give such clean backgrounds in the toned image, it is common to bias a development electrode to a point between the minimum and maximum potentials of the electrostatic ,pattern and close to the potential of the portion not to be toned. This discourages toner particles from being attracted to the background portion of the image leaving it clear. Thus, if the highest potential portions of the pattern are to be toned, the bias would be slightly above the lowest potential portion; if the lowest potential portion is to be toned, the bias would be slightly below the highest potential portion.

With electrophotographically produced charge patterns, different densities or reflectivities of the documents copied (as when documents of different background color are being copied) give different degrees of charge dissipation during the exposure step. Similarly, increased scumming of the pattern forming surface or a reduction in illumination can increase the background charge. To assure low-density background copies for a wide variety of conditions, it is desirable to provide electrographic apparatus with means for adjusting the electrical potential of the development electrode to such a level as to urge toner away from the background areas even though the charge in the background areas changes abruptly.

US. Pat. No. 2,956,487 discloses apparatus for continuously sensing the charge associated with a latent electrostatic image upstream from a single magnetic brush development station. A signal derived from such sensing continually biases a development electrode in accordance with the level of sensed charge at a time when that portion of the charge image responsible for such sensed charge passes the development electrode. The electrode is spaced opposite the magnetic brush on the other side of the photoconductor bearing the sensed charge.

Summary of the Invention In applying a system in which the bias on a development electrode is varied according to an instantaneously sensed. charge pattern, to apparatus using a long development zone, a problem of controlling the bias for the correct portion of the charge pattern is encountered. The problem is most severe when an abrupt change in the charge pattern is experienced. It is generally impractical to vary the bias from one portion of a long development zone to another, especially if the development electrode is a magnetic brush carrier which .is spread continuously or intermittently over the zone.

I have found that the best results are obtained in combining these two seemingly incongruous systems, if the bias on the development electrode is adjusted for a sensed characteristic of that portion of the charge pattern passing the last part of the development zone.

Accordingly to a preferred embodiment of the invention, an induction plate occupies a closely spaced relationship with respect to the path along which an electrostatic image-bearing element travels. At least a portion of the electrostatic image induces a proportional charge on the plate. Circuitry continuously samples the charge induced on the plate and applies a corresponding electrical bias to a two magnetic brush development station, of the type described above, The bias corresponding to the sensed charge is applied at a time when that portion of the electrostatic image which induced such charge passes the last point of toner application, for example, the last point of contact with the second magnetic brush. Pure time delay circuitry is provided to delay the application of the electrical bias to the development electrode until that portion of the electrostatic image responsible for such biasing signal is passing the last part of the development electrode.

Therefore, an object of the invention is to minimize retention of toner in background areas in developing electrostatic images, thereby giving toned images or patterns having high contrast and clean background.

A further object is to obtain such clean backgrounds despite variations in charge density in the background areas of the electrostatic image and despite the use of a developing mechanism having a lengthy development zone and a lengthy equipotential development electrode.

The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.

Brief Description of the Drawing In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawing which is a schematic illustration of a portion of an electrographic machine having an electrographic magnetic brush development apparatus incorporating an automatic sensor in delayed communication with a development electrode.

Description of the Preferred Embodiment The present invention relates to an apparatus for developing or toning an electrostatic image or charge pat tern. Although the invention is not restricted to use with any particular mechanism for obtaining the charge pattern, it is particularly useful with apparatus in which the patterns are produced by electrophotography. Accordingly, electrophotographic apparatus is shown, in part, in the drawing. Because electrophotographic apparatus is well known, the present description will be directed in particular to elements forming part of, or cooperating more directly with, the present invention, and elements not specifically shown or described may take various forms well known to those skilled in the art.

In the drawing, an electrographic recording element 2 is driven through a series of electrophotographic stations by a suitable drive means, here depicted illustratively as rollers 3. The recording element 2 includes a support 8, a conductive layer 5, and a photoconductive layer 6. The photoconductive layer has a surface 7 capable of holding a charge in the dark.

The element 2 is driven by rollers 3 to move surface 7 through a path in which it passes charging station 10 where a portion of the surface 7 receives a uniform electrostatic charge 11 (shown, for the purposes of illustration, as a negative charge) from a corona discharge source or the like. Upon being uniformly charged, the charge-bearing surface is advanced past an exposure station 12 where it is imagewise exposed to actinic radiation in accordance with the light and dark areas of an original document. Such imagewise ex posure serves to selectively dissipate the uniform charge on the photoconductive surface to form an electrostatic latent image 13 corresponding to the original document. Development of the electrostatic image is accomplished as that portion of the recording element bearing the image passes the development station 14 where it is subjected to an electrographic developer. Such a developer commonly comprises a mixture of toner and carrier particles, the latter serving to carry the toner particles by triboelectric forces between them. During the development process, the toner particles are separated from the carrier particles by the stronger forces associated with the electrostatic image and are deposited on the surface of the photoconductive belt to form a toner image 15.

The toner image can be used in a number of ways, for example, it can be transferred to a paper receiving sheet on which the toner can be permanently fixed, or can be fixed directly to the charge carrying surface 7, by appropriate means, not shown.

Development station 14 includes a trough 33 for containing a development mix 34, and conventional magnetic development brushes 36 and 38 which are rotatably mounted in trough 33 and serve to apply the development mix to surface 7 to form a developed or toned image 15. Each of the development brushes 36 and 38 can be constructed in a manner well known in the art, see, for example, US. Pat. No. 3,543,720, Drexler et al. For example, each of brushes 36 and 38 include a rotatable, non-magnetic cylinder with a fixed magnetic field producing means inside. The field producing means attracts magnetic carrier particles to the cylinder to form bristles. Rotation of the cylinder brushes the bristles across the charge pattern 13. Toner triboelectrically coated on the carrier particles is then deposited on the charge pattern according to the relative attraction of the carrier and the pattern. Trough 33 is arranged adjacent the path along which the recording element 2 travels, being spaced therefrom such that the bristles of the development brushes contact the surface 7 at all times.

As is well known, a magnetic development brush, in addition to functioning as a means for applying toner to an electrostatic image, can also function as a development electrode and thereby assist in the development of large solid image areas. Development electrodes have a tendency to permit deposition of toner in background areas, However, by controlling the electrical potential or bias of a development electrode, it is possible to effectively neutralize the charge in the background areas of the electrostatic image while, at the same time, providing a field between the solid image areas requiring development and the develop ment brush which is sufficient to attract toner particles to such solid areas.

For a number of possible reasons, the charge density in the background may vary by as much as several hundred volts. During the exposure of the recording element to a document of high density, little of the uniform charge previously applied to the recording element is dissipated. For example, with some photoconductors a green background original will give a much higher background voltage or charge density than will a white background original. The development electrode bias must be high to neutralize such charge. Thus,

the optimum electrode bias is directly proportional to the background voltage, the denser the background of the original, the higher the electrode bias. Typically, an offset bias (e.g., 50 volts) is applied to the development electrode to render the development electrode more attractive to toner particles than the background areas of the charge image. In this manner, the background areas of the charge image tend to be repulsive to toner particles.

To control the electrical potential of the development brushes in accordance with the background density of the document being copied, there is provided an induction plate 40 which is positioned in close proximity to the path along which the recording element travels, upstream from the developing stations. Induction plate 40 comprises a plate of highly conductive material, such as copper or aluminum, preferably having dimensions smaller than the overall dimensions of the electrostatic image. However, the induction plate size is not critical and could very reasonably be made to extend the full width of the recording surface. Plate 40 is positioned parallel to the photoconductive surface of the recording element and spaced sufficiently close thereto to receive an induced charge from the electrostatic charge pattern borne by such surface as it passes by. Preferably, plate 40 is located between the lateral edges of the recording element. Thus, the charge induced on plate 40 is, at any given time, proportional to the average instantaneous charge borne by the portion of the recording element which is directly opposite the induction plate. By way of the circuitry described hereinbelow, the voltage induced on induction plate 40 is continuously sensed during the passage of the electrostatic image past the induction plate, and a biasing voltage, proportional to the instantaneous level of sensed charge, is applied to the development brushes. Obviously, the bias voltage will have to be delayed until the sensed charge is passing the development station.

As can be seen from the drawing, the charge pattern 13 is first contacted by the toner applying means at an upstream position 51. It is continuously or intermittently contacted by toner and carrier until it reaches a downstream position 52 giving a lengthy development zone. The carrier being conductive will be substantially equipotential throughout. But the image will vary in character over the portion being toned. For example, a green background document may be copied immediately after a white background document. I have found that good results can be obtained if the delay is timed so that the bias adjustment corresponds to the last contact area between the toner applying mechanism and the photoconductive surface. For example, the biasing voltage can be timed to be applied when the electrostatic charge pattern which produced the induced charge is passing the last brush in a two magnetic brush development station, downstream position 52.

This delayed bias voltage produced better copies than those obtained from development electrodes biased in a conventional manner. Even in apparatus in which the first brush 36 and developer between the brushes substantially completes development of the electrostatic charge pattern on the recording element 2, the final touch of development over the second brush 38 has a profound effect on the quality of the final copy. The second brush 38, with bias timed to be applied thereon, essentially fine tunes the development process. If black image areas are too light, it adds toner to them, and if the background has picked up toner it cleans it off.

An shown in the drawing, the voltage induced on plate 40 due to the passage of electrostatic image 13 in close proximity thereto is amplified by an amplifier 42. Amplifier 42 can be of any conventional type suitable for the purpose of amplifying the output from induction plate 40, for example, an FET input operational amplifier. The output of amplifier 42 is used to control the output voltage of a bias amplifier 44 which is directly connected to the magnetic brush 38 or to housing 33. To synchronize the application of the appropriate bias voltage with the passage of that portion of the electrostatic image responsible for such bias voltage, the output of amplifier 42 is passed through a pure time delay circuit. The embodiment shows a pure time delay circuit 46 based on Pade' approximants. Other suitable approximants may also be used.

The theory of operation of the pure time delay circuitry schematically illustrated in the drawing has been disclosed by Per A. Holst in an article entitled Pade Approximations and Analog Computer Simulations of Time Delays, Simulation, Simulation Councils, lnc., June 1969, pps. 277-290, and by C. L. Johnson, in a text entitled Analog Computer Techniques", 2nd Ed., McGraw Hill, Inc., New York, pps. 116-125. Bias amplifier 44 drives the developer brushes to a bias level which is proportional to its input signal coming from pure time delay circuit 46. The bias amplifier 44 can be 4 of any suitable conventional design, for example, an operational amplifier driving high voltage control transistors which connect between the bias supply voltage and the bias amplifier output. For example, although positive toning of the charged image areas is described above, the invention will also work with reverse development systems in which the discharged areas are toned and the charged, image areas are kept clean.

The invention has been described in detail with particular reference to the preferred embodiment thereof, but itwill be understood that variations and modifications can be effected within the spirit and scope of the invention. I claim: 1. ln electrographic apparatus of the type including a development station and means for advancing a surface bearing an electrostatic charge pattern along a predetermined path past the development station and in which said development station includes at least two magnetic brushesarranged to continuously contact the surface as it moves therepast and means for applying an electrical potential to said brushes, the improvement comprising:

means for instantaneously varying the electrical potential applied to said brushes to correspond uniquely to the charge characteristics of the individual charge pattern portion instantaneously passing the last contact area between the magnetic brushes and the surface. 2. The invention according to claim 1 wherein said controlling means comprises:

sensing means for continuously sensing charge characteristics of each of the transverse segments of the electrostatic charge pattern passing a predetermined location upstream of the development station and for generating successive electrical signals respectively proportional to the charge characteristics of each of the transverse segments; and

means for sequentially receiving said signals and for ith a characteristic of the charge on that seg sequentially applying to said brushes a bias voltage ment of the surface then passing the last contact which varies in magnitude in proportion to said area of the downstream position. electrical signals, said bias voltage being applied in 7. The improvement according to claim 6 wherein a timed relationship which is related to the rate id toner applying means in lude means for applying movement of said sen ed charg p rn in a m ntoner to said pattern continuously from at least said upner Such that the instantaneously pp bias Volt stream position to said downstream position and said is proportional to the sensed Charge Fharacter' development electrode is positioned to control develistics of the segment of the electrostatic image that Opmem f at least Said upsimam i i to d is passing the last downstream contact area between the brushes and the surface. 3. The invention according to claim 2 wherein said signal receiving and voltage applying means comprises:

time delay means for receiving an electrical signal 10 downstream position.

8. Electrographic apparatus including:

means for advancing a surface bearing a charge pat tern through a path;

means for applying toner to the pattern as the surface generated by 52nd sfimsmg means delaymgthg is advanced, said means applying toner over a elec trigal transmissiondof that signal for a time lengthy development Zone; g l to the ti movemfena of surfac e means for applying an instantaneously variable elec- (mung t e Sense segment 9, t e e ctrostzfnc trical bias to said toner applying means to create a charge pattern from a first position at said sensing field Controlling Such toner application means to a second position at the last contact area betw th b h d th f d means for continuously sensing a characteristic of 6 an e bur an said pattern as a segment of said pattern passes a bias means for receiving said delayed signal from said position upstream from said toner applying means; time delay means, for generating a bias voltage corand responding to said sensed characteristic and for means responsive to said sensing means for instantaelectrically coupling said bias voltage to the devel- Opmem Station neously controlling said bias exclusively in propor- 4 The invention'according to claim 3 wherein Said tion to said sensed characteristic, said controlling time delay means comprises a pure time delay circuit w m cludmg means for delaymg the based on Pade. approximams of said bias control until the sensed segment of said 5. The invention according to claim 4 wherein said pattern reaches the last pomon of Bald develop' sensing means comprises: mam Zone I 4 an induction plate for generating said signal, which Electrogrlphlc zlpparatus gplate is mounted in close proximity to the surface; mean? for advancmg a Surface bearmg an gleam and static charge pattern through a path; amplifier means, having an input connected to said mfians for applymg toner tothe Pf as the surface induction plate and an output connected to said advanced- Sald tneans lncludlng I time delay means, for amplifying said signal. at least two magnetic brushes arranged in tandem, 6. in apparatus inciuding; means for applying a common, instantaneously means f advancing a Surface bearing an electro. variable, electrical bias to both of said brushes to Static charge pattern through a path; 40 control the application of toner to the pattern, means f appiying toner to the pattern as i means for sensing a characteristic of each segment of face is advanced through said path, said means 531d Pattern as it Passes a Posltlon upstream from being of the type which applies toner at at least an 581d toner pplying "l a upstream o ition nd a downstream iti id means responsive to said sensing means for controld t iti b i th l t o itio at ling said bias for each such segment according to hi h t n i li d to the att said sensed characteristic of such segment, said a development electrode; and controlling means including means for delaying the means for applying a bias to said development elecapplication of the bias for each segment until that trode; the improvement comprising: segment is passing the last position at which toner means for instantaneously controlling the bias on is applied to said pattern.

said development electrode to correspond solely 

1. In electrographic apparatus of the type including a development station and means for advancing a surface bearing an electrostatic charge pattern along a predetermined path past the development station and in which said development station includes at least two magnetic brushes arranged to continuously contact the surface as it moves therepast and means for applying an electrical potential to said brushes, the improvement comprising: means for instantaneously varying the electrical potential applied to said brushes to correspond uniquely to the charge characteristics of the individual charge pattern portion instantaneously passing the last contact area between the magnetic brushes and the surface.
 2. The invention according to claim 1 wherein said controlling means comprises: sensing means for continuously sensing charge characteristics of each of the transverse segments of the electrostatic charge pattern passing a predetermined location upstream of the development station and for generating successive electrical signals respectively proportional to the charge characteristics of each of the transverse segments; and means for sequentially receiving said signals and for sequentially applying to said brushes a bias voltage which varies in magnitude in proportion to said electrical signals, said bias voltage being applied in a timed relationship which is related to the rate movement of said sensed charge pattern in a manner such that the instantaneously applied bias voltage is proportional to the sensed charge characteristics of the segment of the electrostatic image that is passing the last downstream contact area between the brushes and the surface.
 3. The invention according to claim 2 wherein said signal receiving and voltage applying means comprises: time delay means for receiving an electrical signal generated by said sensing means and delaying the electrical transmission of that signal for a time equivalent to the timed movement of the surface bearing the sensed segment of the electrostatic charge pattern from a first position at said sensing means to a second position at the last contact area between the brushes and the surface; and bias means for receiving said delayed signal from said time delay means, for generating a bias voltage corresponding to said sensed characteristic and for electrically coupling said bias voltage to the development station.
 4. The invention according to claim 3 wherein said time delay means comprises a pure time delay circuit based on Pade approximants.
 5. The invention according to claim 4 wherein said sensing means comprises: an induction plate for generating said signal, which plate is mounted in close proximity to the surface; and amplifier means, having an input connected to said induction plate and an output connected to said time delay means, for amplifying said signal.
 6. In apparatus including: means for advancing a surface bearing an electrostatic charge pattern through a path; means for applying toner to the pattern as said surface is advanced through said path, said means being of the type which aPplies toner at at least an upstream position and a downstream position, said downstream position being the last position at which toner is applied to the pattern; a development electrode; and means for applying a bias to said development electrode; the improvement comprising: means for instantaneously controlling the bias on said development electrode to correspond solely with a characteristic of the charge on that segment of the surface then passing the last contact area of the downstream position.
 7. The improvement according to claim 6 wherein said toner applying means includes means for applying toner to said pattern continuously from at least said upstream position to said downstream position and said development electrode is positioned to control development from at least said upstream position to said downstream position.
 8. Electrographic apparatus including: means for advancing a surface bearing a charge pattern through a path; means for applying toner to the pattern as the surface is advanced, said means applying toner over a lengthy development zone; means for applying an instantaneously variable electrical bias to said toner applying means to create a field controlling such toner application; means for continuously sensing a characteristic of said pattern as a segment of said pattern passes a position upstream from said toner applying means; and means responsive to said sensing means for instantaneously controlling said bias exclusively in proportion to said sensed characteristic, said controlling means including means for delaying the application of said bias control until the sensed segment of said pattern reaches the last portion of said development zone.
 9. Electrographic apparatus including: means for advancing a surface bearing an electrostatic charge pattern through a path; means for applying toner to the pattern as the surface is advanced, said means including at least two magnetic brushes arranged in tandem, means for applying a common, instantaneously variable, electrical bias to both of said brushes to control the application of toner to the pattern, means for sensing a characteristic of each segment of said pattern as it passes a position upstream from said toner applying means; and means responsive to said sensing means for controlling said bias for each such segment according to said sensed characteristic of such segment, said controlling means including means for delaying the application of the bias for each segment until that segment is passing the last position at which toner is applied to said pattern. 